The present invention provides improved zinc sputtering targets of the type commonly used in depositing thin dielectric layers via sputtering, e.g., in the manufacture of low emissivity film stacks.
Sputtered coatings of various materials are used in a wide variety of applications. One of the most common applications for thin sputtered films is in making thin optical films. For example, low emissivity films commonly applied to window glazings comprise a series of layers formed of metals and dielectrics such as metal oxides or nitrides. In most commercial production, such thin films are applied by sputtering a target of a specific material in a controlled atmosphere to produce the desired film composition. If a metal layer is being applied, (e.g., a layer of silver), the metal target will typically be sputtered in an inert atmosphere, such as argon. Metal oxide and nitride films can be applied either by sputtering a target of the desired oxide or nitride or by sputtering a metal target in the presence of oxygen or nitrogen and allowing that sputtered metal to react to form the desired oxide or nitride.
One metal oxide layer which is common in the formation of low emissivity coatings is zinc oxide. The popularity of zinc oxide in such film stacks can be attributed largely to the relatively high sputtering rate for zinc targets in an oxygenating atmosphere. This rapid sputtering rate speeds production, thereby reducing the cost of manufacturing the low emissivity film stack. One disadvantage of reactive sputtering of a zinc target to apply coating of ZnO, however, is the tendency of such targets to produce xe2x80x9cfireflies.xe2x80x9d It is believed that these fireflies are particles of the zinc target which are knocked off during sputtering and are oxidized as they pass through the reactive gas. During optimal sputtering conditions, the zinc from the target is sputtered on almost an atom-by-atom basis, and such energized zinc atoms will react with the oxidizing atmosphere as they proceed toward the substrate to be coated. The larger particles associated with fireflies do not always oxidize and can land on the substrate as incompletely oxidized particles. Given the rather thin coatings used in low emissivity film stacks, such large particulate inclusions can have a noticeable adverse impact on the quality of the finished product.
The exact mechanism by which fireflies are created when sputtering a zinc target is not fully understood. While other target materials could also yield fireflies, in normal commercial production parameters this problem is significantly more pronounced with zinc targets. It has been theorized that zinc is more susceptible to this firefly phenomenon due to the relatively brittle, friable nature of zinc as opposed to other, more malleable materials. Zinc also fairly readily oxidizes, which may tend to cause a layer of zinc oxide to buildup on the target. This friability of the zinc, in combination with the formation of a thin crust of zinc oxide, may enhance the tendency of the target to shed larger particles rather than uniformly sputtering atomic zinc to yield a homogenous, carefully controlled film.
In U.S. Pat. No. 4,610,771 (the teachings of which are incorporated herein by reference), Gillery et al. suggest the use of a sputtering target which comprises an alloy of zinc and tin. Gillery et al. claim that this alloy yields a superior film, but focus fairly little on the nature of the target itself, other than stating that it is an alloy having a specified composition. Gillery et al. also focus on the use of zinc and tin to the exclusion of other combinations, pointing out that other possible materials to alloy with zinc (e.g. bismuth) are undesirable for various reasons.
The present invention provides a target for use in sputtering, a method of making such a target, and a method of depositing a film on a substrate. In accordance with one embodiment of the invention, a target comprises a backing adapted to be operatively connected to a sputtering power source and an outer layer of a sputterable material carried by the backing. This sputterable material comprises a mixture of zinc and a second metal having a melting point less than that of the zinc. The zinc and the second metal are present in the sputterable material in metallic form and are arranged as discrete volumes of the second metal in a matrix of zinc. In one preferred adaptation of this target, the second metal is selected from the group consisting of tin, bismuth and indium or alloys or admixtures thereof. The target desirably comprises no less than 80% zinc on a weight basis, and optimally comprises no less than 91% wt. zinc.
Another embodiment of the invention provides a method of forming a target for magnetron sputtering. In accordance with this method, a backing having an exterior surface is provided. Zinc metal and a second metal are simultaneously plasma sprayed to create an outer layer of a sputterable material carried by the backing. The second metal used in this method has a melting point less than that of the zinc. If so desired, the second metal may be selected from the group consisting of tin, bismuth, and indium or alloys or admixtures thereof. In one particularly preferred method, the zinc and the second metal are delivered from discrete sources, whereby the outer layer of sputterable material comprises discrete areas of the second metal in a matrix of zinc.
The present invention also provides a method of depositing a film on a substrate. In accordance with this method, a target is provided. This target comprises a backing and an outer layer of a sputterable material carried by the backing The sputterable material comprises a mixture of zinc and a second metal having a melting point less than that of the zinc. The zinc and the second metal are present in the sputterable material in metallic form and arranged as discrete volumes of the second metal in a matrix of zinc. The target and a substrate are placed in a sputtering chamber and the target is connected to a power source. Power is applied to the target while maintaining in the sputtering chamber a reactive atmosphere comprising oxygen. This deposits on surface of the substrate a film comprising oxides of zinc and the second metal.
Preferably, sputtering of the target consumes the zinc and the discrete volumes of the second metal, thereby exposing areas of the second metal previously covered by zinc and exposing areas of zinc previously covered by the second metal.